In order to acquire objects or surfaces, use is often made of methods which successively scan and, in the process, record the topography of a structure such as e.g. a building. Here, such a topography constitutes a related sequence of points, which describes the surface of the object, or else a corresponding model or description of the surface. A conventional approach lies in carrying out a scan by means of a laser scanner which in each case acquires the spatial position of a surface point by virtue of the distance to the targeted surface point being measured and this measurement being linked to the angle information relating to the laser emission. From this distance and angle information, the spatial position of the respectively acquired point can be determined and the surface can be measured continuously. In many cases, an image is also recorded by means of a camera in parallel with this purely geometric acquisition of the surface, which camera also provides further information, for example in respect of the surface texture, in addition to the visual overall view.
Thus, for example, WO 97/40342 describes a method which records a topography by scanner systems set up in a stationary manner. A fixed set up point is selected for these systems, which set up point serves as a base for a scanning process brought about by motors. The three-dimensional spatial information of the respective surface point can be derived from the distance to the measured point, the angle position at the time of the measurement and the known location of the scanning device. Here, the scanner systems are designed specifically for the object of acquiring the topography and scan a surface by moving the scanner system or by modifying the beam path.
Other methods use mobile systems, which scan a structure to be acquired by a movement of the scanner system, or which support or complement the scan. Such systems are particularly suitable for acquiring linear or linearly drivable structures such as e.g. track installations, roads, tunnel systems or airfields.
Such acquisition processes known from the prior art provide images or topographic data which essentially represent the information about the spatial distribution or arrangement relationship of surface points. Optionally, additionally recorded images enable the derivation of further information. As a result, the structure and the profile of the surface can be reconstructed comparatively well. However, the lack of qualitative specifications about the type and composition of the surface, in particular in view of the inner structure or composition, is disadvantageous. Thus, images recorded parallel to the scanning usually enable the identification of different brightness values. Furthermore, EP 1 759 172 describes a scanner system and a method for acquiring surfaces in a spectrally resolved manner, which provides a derivation of surface properties from the information obtained thereby.
Such laser scanners according to the prior art enable a user to acquire large surfaces and objects completely, and optionally with additional object information, while expending relatively little time—depending on a desired point-to-point resolution. Here, laser scanners are typically configured in such a way that primarily point clouds with a large number of measurement points can be acquired and this acquisition is brought about with a sufficient accuracy. Since laser scanners do not have a targeting apparatus for highly precise targeting of a target, this accuracy of the point coordinates derivable in the process does not meet the high geodetic accuracy standards, as have been established for e.g. modern surveying instruments, in particular for total stations or theodolites.
In general, modern total stations have a compact and integrated design, wherein coaxial distance measurement elements and computer, control and memory units are usually available in an instrument. Depending on the configuration level of the total station, a motorization of the targeting or sighting apparatus and—in the case where retroreflectors (for example an all-round prism) are used as target objects—means for automated target search and tracking can moreover be integrated. As a human-machine interface, the total station can comprise an electronic viewer/control unit—generally a microprocessor computer with electronic data storage means—with a display and input means, e.g. a keyboard. The measured data acquired by electro-sensory means are fed to the viewer/control unit such that the position of the target point can be established, optically displayed and stored by the viewer/control unit. Total stations known from the prior art can furthermore comprise a radio data interface for establishing a radio link to external peripheral components such as e.g. a portable data acquisition instrument which, in particular, can be embodied as a data logger or field computer.
For sighting or targeting the target point to be measured, generic geodetic surveying instruments have a telescopic sight, such as e.g. an optical telescope, as a sighting apparatus. In general, the telescopic sight can be rotated about a vertical axis and about a horizontal tilt axis relative to a base of the surveying instrument such that the telescope can be aligned on the point to be measured by pivoting and tilting. In addition to the optical viewing channel, modern instruments can comprise a camera, which is integrated into the telescopic sight and, for example, aligned coaxially or in parallel, for acquiring an image, wherein the acquired image can, in particular, be depicted as a live image on the display of the viewer/control unit and/or on a display of the peripheral instrument—such as the data logger—used for remote control. Here, the optical unit of the sighting apparatus can comprise a manual focus—for example a set screw for changing the position of a focusing optical unit—or have an autofocus, wherein the focus position is changed by e.g. servomotors. By way of example, such a sighting apparatus of a geodetic surveying instrument is described in EP 2 219 011. Automatic focusing apparatuses for telescopic sights of geodetic instruments are known from e.g. DE 197 107 22, DE 199 267 06 or DE 199 495 80.
Since target objects (e.g. the plumb rods with target marker, such as an all-round prism, which are usually used for geodetic purposes) cannot be targeted sufficiently precisely with the naked eye on the basis of the sighting apparatus despite the 30-fold optical magnification often provided, conventional surveying instruments in the meantime have as standard an automatic target tracking function for prisms serving as target reflector (ATR: “Automatic Target Recognition”). For this, a further separate ATR light source—e.g. a multimode fiber output, which emits optical radiation having a wavelength in the range of 850 nm—and a specific ATR detector (e.g. CMOS area sensor) sensitive to said wavelength are conventionally additionally integrated in the telescope. By way of example, EP 2 141 450 describes a surveying instrument with a function for automatic targeting of a retroreflecting target and with automatic target tracking functionality.
Additionally, scanning functions can be integrated into total stations or theodolites as additional functions. By way of example, WO 2004/036145 has disclosed a geodetic measurement instrument which emits a laser beam for measuring the distance from the position thereof from within the acquired region. Such measurement instruments can likewise be modified for the scanning acquisition of surfaces or operated without modification. Motorized theodolites or total stations constitute an example for this.
Using such modern surveying instruments, the coordinates of target points to be measured can be determined with a very high geodetic precision. To this end, the laser beam must initially be aligned very accurately on the target, and the distance to the target and alignment of the laser beam have to be determined in this targeting state. Subsequently, it is possible to derive a position of the target (at least relative to the surveying instrument). However, a disadvantage here is that a large-area topographic object survey using e.g. a total station therefore means a disproportionately high expenditure of time—compared to a measurement process of the laser scanner on the object—in particular as a result of the mass of the telescope to be aligned.
Depending on a surveying object addressed, a surveyor can therefore require either a laser scanner or a total station/theodolite for working on the object. Moreover, for example, it may be necessary within the same survey to scan a surface and precisely determine a point of a single target point situated on the surface or a different object. In general, two differently designed instruments, namely a laser scanner for scanning surfaces and a total station for an accurate geodetic determination of a target point position, therefore have to be available. This constitutes a great disadvantage, in view of the number of instruments and peripheral components to be carried around or kept available, in view of the acquisition costs connected therewith and in view of the learning outlay for proper operation of both instruments.